1. The proteids:

2. The hydro - carbons, or fatty matters, carbon, hydrogen, and oxygen, in a certain proportion, are also necessary for nutrition; and these may be obtained also from both animals and vegetables.

3. Carbo-hydrates, also carbon, hydrogen, and oxygen, but in different proportions from those of the preceding class, not absolutely essential to life like the proteids, although they are most desirable elements of food. They are largely furnished by the vegetable kingdom, consisting chiefly of the starches of all grain, roots, and tubers, with the sugars and the gums; in milk they exist as milk sugar, or "lactose."

From animals;

From vegetables.

2. The hydrocarbons.

3. The carbohydrates.

4. Lastly, there are certain products no less essential than the first class, all belonging solely to the inorganic or mineral kingdom, namely, water in large quantity, with salts of soda, lime, magnesia, potash, phosphorus, iron, and other metals. All these must be present in the food supplied, and are obtained in most forms of food from both animal and vegetable sources (see note, p. 41).

Meantime, as a result of chemical changes which take place in every portion of the body through the assimilation of all these elements, chiefly by oxidation, heat is produced; while not only the proteids, but also the fatty matters and the carbo-hydrates of our food, as well as the inorganic constituents, are utilized in the repair of wasted tissues.

Thus we may regard the human body as a complex and highly organized machine, adapted to execute work of varied but specific kind, but one which is self-supplying by means of food, and also automatically regenerates itself in order to maintain a condition of good repair.

Thus, any material which is competent to supply these requirements is a complete or perfect food. Examples of complete food exist in milk and the egg, sufficing as these do for all the wants of the young animal during the period of early growth. Nevertheless, a single animal product like either of the two named, although complex in itself, is not more perfect than an artificial combination of various simpler substances, provided the mixture (dish or meal) contains all the elements required in due proportion for the purposes of the body.

4. Inorganic elements: water, salts, etc.